Field Level Security gives permissions to administrators to restrict user’s access to view and edit some specific fields.
At the object level, we can give permission on what users can do with salesforce records like users can read, edit, create, delete etc. Object permissions are given at object level. To restrict certain sensitive fields from your users and don’t want to display these fields to all users, we can do it by using field level security.
For example, your Manager wants to keep approval of the leave field to select employees, you can restrict the user's access by using field level security on that particular field.
Page Layouts and Field level security settings determine which fields a user sees.
There are two ways to set field level security
From Setup Enter Administer--> Security Controls--> Field Accessibility
Click on Field Accessibility.
Now you get the list of object records available in your organization. Select one of the objects whose field to be modified.
Under Choose your view section we will have view by fields and view by profiles options.
If we select View By Fields option, we can access field level security for only one field.
Click on View by fields and select the required field from drop down, click on field access with suitable profile and edit the field level security.
Click on View by Profiles, Select Profile from drop down and edit the field level security by clicking on the required field name.
In Another Way,
From Setup enter Administer--> Manage Users--> Profiles
Click on Required Profile
Under Field level Security Section, click on view for desired field and edit the field level security for that field.
Only two options Read access and Edit access are available.
Click on Save
Go to the field you want to set the field level security
Click on Set Field Level Security Button
Change the visible and read only options for the particular profile
Click on save.